The present invention relates to a borer head for drilling boreholes in rock structures.
The borer head of the type under consideration comprises a central shaft coupled to a simple driving shaft or rod and on which at least one tool holder, provided with cutters, is driven and rotated by means of a planetary gearing.
One of such borer heads is disclosed in the U.S. Pat. No. 3,771,611. In this conventional rock borer head the cutters are formed as cutting discs spaced from each other by angle 120.degree., the cutting discs being driven from the central shaft in a counter direction therewith and with reduced number of revolutions. The cutting discs operate in the direction of the drive torque, and for this purpose a drive translation between the central shaft and the driving shafts carrying the cutting discs is attained by a respectively formed planetary gearing. The borer head of such a construction must produce in the rock structure being drilled a respectively high cutting output. Then the cutting tool, due to a counter rotation of the shafts and due to a gear ratio with a highest torque, generates in the rock structure a reaction torque which assists in a revolving motion of the housing of the borer head about the central shaft. Thus this rotation moment assistance occurs, which sets the non-directly driven borer head housing in a required revolving motion. Thus the gear ratios, the number of the cutters on individual circular tracks of the cutting discs and the size of the circular surface of the borehole being drilled can be determined in dependence from each other.
Tests have surprisingly shown that the head borer of the conventional design can not ensure a satisfactory motion of the tool holder as well as satisfactory cutting speeds of the cutters. It has been determined that the tool holder can freely rotate with high number of revolutions and then turn about the central shaft with non-satisfactory rotation. This can be specifically observed from FIG. 2 of U.S. Pat. No. 3,771,611, in which the individual cutters in the proximity of the apex point of the involute track, lying on the wall of the borehole being drilled do not produce a cutting action but rather a rotational motion about themselves. Thus only a slowly and hardly removable step remains in the borehole, which significantly obstructs the feeding of the borer head. With the higher number of revolutions of the tool holder and with reduced drilling output it is impossible, however, to change the counter rotation of the whole borer tool relative to the central shaft.